Polypropylene-based multilayer films are widely used in packaging applications, such as pouches for dry food mixes, pet foods, snack foods, and seeds. Such multilayer films must have the ability to form reliable hermetic seals at relatively low temperatures. In some situations the film must do so in the presence of contamination in the seal region from the contents of the packaging.
U.S. Pat. No. 4,578,316 (Clauson et al.) discloses an oriented film structure having a low film to film coefficient of friction comprising a polypropylene film having on at least one surface a layer of a blend of polypropylene and a member selected from the group consisting of medium density polyethylene, high density polyethylene, and a mixture thereof.
U.S. Pat. No. 4,956,232 (Balloni et al.) discloses a multilayer polypropylene film structure consisting essentially of an outer heat sealable layer (A) formed from a polymer composition consisting essentially of heat sealable resin compounded with slip additives which are incompatible with polypropylene; core layer (B) consisting essentially of an isotactic polypropylene compounded with slip additives which are incompatible with polypropylene; and outer layer (C) consisting essentially of an isotactic polypropylene in the substantial absence of slip additives.
U.S. Pat. No. 5,888,648 (Donovan et al.) discloses a multilayer film which has a composite structure for providing hermetic seals to packages manufactured in a high speed packaging apparatus. The structure of the multilayer film includes a main substrate and a sealant layer. The sealant layer, in turn, includes an intermediate layer which has the primary function of compliance during sealing and a sealing layer which has the primary function of providing adhesive properties to the completed seal. The terms “compliant” or “compliance” refer to the ability of the sealed area of film to deform or conform within the sealing jaws during sealing operations and additionally to elastically and/or plastically deform and diffuse stress throughout the multilayer film substrate subsequent to sealing operations when the seal is subjected to stress.
U.S. Pat. No. 6,312,825 B1 (Su et al.) discloses a multilayer film of the type suitable for packaging applications having oxygen and water vapor transmission barrier properties including a polyolefin core layer containing wax; a first surface layer including a polyolefin copolymer or terpolymer; and a second surface layer including a polar layer having ester functionality.
U.S. Pat. No. 6,326,068 (Kong et al.) discloses a multilayer film that has a composite structure for providing hermetic seals to packages manufactured in a high speed packaging apparatus. The structure of the multilayer film includes layers A/B/C/D. Skin layer A is formed from polypropylene copolymer with melt flow rate greater than one or linear high density polyethylene with melt index greater than one. Core layer B is formed from polypropylene. Intermediate layer C has the primary function of compliance during sealing and sealing layer D has the primary function of providing adhesive properties to the completed seal. The sealing layer D includes an antiblock agent comprising non-distortable organic polymer particles having an average particle size greater than 6 μm.
U.S. Pat. No. 6,534,137 B1 (Vadhar) discloses a two-component laminated multilayer film suitable for use in packaging articles, such as pet food, comprising a first component and a non-heat-shrinkable second component. The first component comprises an outer first film layer, an optional second film layer, and an optional third film layer. The first and third film layers comprise ethylene/alpha-olefin copolymer, while the second film layer is a modified ethylene copolymer. The second component comprises an outer fourth layer, an oxygen barrier fifth layer, and sixth and seventh layers which serve as tie layers and are positioned on either side of the barrier layer. The multilayer film is heat sealable to itself and another film.
U.S. Pat. No. 6,624,247 B1 (Kume et al.) discloses a polypropylene-based film of a resin composition (C) comprising: 40 wt % to 95 wt % of a propylene-based copolymer (A) selected from a propylene random copolymer (i) of propylene with ethylene and/or alpha-olefin and a propylene-based copolymer (ii) composed of 1 wt % to 30 wt % of a component (a) obtained by copolymerizing propylene, an alpha-olefin, and ethylene in a first step; 70 wt % to 99 wt % of a component (b) obtained by copolymerizing those in the subsequent step, in which the copolymerization ratio is different from that in the first step; and 5 wt % to 60 wt % of a polypropylene-ethylene and/or alpha-olefin block copolymer (B) having a xylene soluble component (“CXS”) of 5.0 wt % or more, wherein the CXS has a content of ethylene and/or the alpha-olefin of 14 to 35 molar percent and wherein the heat-seal temperature of the film of the composition (C) is lower by 3° C. or more than those of respective films of the compositions (A) or (B).
U.S. Pat. No. 6,641,913 B1 (Hanyu et al.) discloses a multilayer polyolefin film of the type suitable for packaging applications in which heat seals are formed. The multilayer film comprises a substrate layer formed of a crystalline thermoplastic polymer having an interface surface. A heat-sealable surface layer is bonded to the interface surface of the substrate layer and is formed of a syndiotactic propylene polymer effective to produce a heat seal with itself at a sealing temperature of less than 110° C. The multilayer film may be biaxially-oriented. In the production of the multilayer film, a crystalline thermoplastic polymer is extruded and formed into a substrate layer film. A second polymer comprising a syndiotactic propylene polymer which is effective to form a heat-sealable surface layer is extruded separately to form a surface layer that is thereafter bonded to the interface of the substrate layer at a temperature within the range of 150° C. to 260° C.
U.S. Pat. No. 6,682,822 B2 (Cretekos et al.) discloses a multilayer polymeric film comprising a core layer comprising thermoplastic polymer and optionally an antistatic agent; a printable or sealable functional layer; and an additional layer on the opposite side of the core layer comprising polypropylene homopolymer, a solid antiblock, and silicone oil.
U.S. Pat. No. 7,537,829 B2 (Pellingra et al.) discloses a multilayer film suitable for packaging and having improved sealing properties. The film comprises a core layer comprising a core polymer and at least one soft polymer; a first tie layer comprising a soft polymer and, optionally, a tie layer polymer; and a sealant layer on the first tie layer opposite the core layer comprising a Zeigler-Natta or metallocene catalyzed polypropylene homopolymer, ethylene-propylene copolymer, propylene-butylene copolymer, ethylene-butylene copolymer, ethylene-propylene-butylene terpolymer, ethylene vinyl acetate, or blend thereof. The seal strength of this film is disclosed to be greater than 600 grams per inch for a seal formed at a temperature of at least 93.3° C. The publication U.S. 2011/0129681 A9 (Rehkugler et al.), related to U.S. Pat. No. 7,537,829, discloses a sealable multilayer packaging structure comprising a first substrate; and a second substrate comprising polymeric film including a core layer with certain specifically described properties, a tie layer with certain specifically described composition, and a sealant layer.
Though each of the above disclosed films represent various points related to packaging films, none of the above disclosed films combine desired improvements in processability, seal strength, reduced seal temperature, and coefficient of friction needed for some of today's challenging packaging operations. Opportunities exist for polymer films to replace other packaging substrates, such as paper and foil, in many temperature-sensitive packaging operations. The present invention meets these and other needs.